No, Terminal 5 does actually close at night and there is a clear-down search by HAL staff after the final departure (usually to Abuja at 22:30). Anyone found airside is turfed out invited to go to the coffee bar in Arrivals landside. They do very occasionally keep airside open all night, when there is bad weather, but I don't think they've needed to do that for a couple of years now. Some Border Force agents are kept available all night long just in case there is an unexpected arrival (e.g. diversion).